This multi-PI R21 proposal describes a high-risk, high-impact project that introduces a novel method for recording taste-evoked activity in gustatory afferent neurons. The work represents a major new direction for the PIs as well as for the chemical senses community, and thus is ideally suited for the R21 mechanism. The project will introduce a novel recording technique to produce new data on the taste sensitivities and transmitter systems of large ensembles of geniculate ganglion neurons. We will employ a new strain of genetically-engineered mice that express a fluorescent functional reporter, GCaMP3, selectively in sensory (including gustatory) ganglion neurons. We propose to develop a new recording technique to image taste-evoked activity in fluorescent geniculate ganglion neurons in anesthetized mice. Geniculate neurons will be imaged with scanning laser confocal microscopy and changes in fluorescence quantified to simultaneously measure activity in large ensembles of neurons with single cell resolution. Ganglion cells will be excited by prototypic sweet, bitter salty, sour, umami and fat taste stimuli, delivered in the oral cavity. We will measure the breadth of tuning, concentration-response relations and entropy for gustatory geniculate ganglion cells (Aim 1). Functionally characterized neurons will be isolated and single cell RT-PCR will be carried out to examine the neurotransmitter systems employed by functionally distinct taste neurons (Aim2). Successful completion of the two aims will have 4 outcomes: first, we will have developed a completely new method for recording afferent sensory activity in large numbers of gustatory neurons~ second, we will accumulate a large database of response profiles for gustatory afferent axons, providing a comprehensive catalog of their breadth of tuning and entropies~ third, we will determine whether different classes of sensory neurons (generalists~ specialists~ sweet-, sour-sensing, etc.) have different molecular expression profiles, such as distinctive transmitter systems, transcription factors, and so forth~ and finally, whether there i a unique class of fat-sensing geniculate ganglion neurons. The data will have implications for how taste is coded by sensory afferents (e.g., labeled line vs, combinatorial coding) and will tremendously increase our understanding about how sensory neurons process gustatory signals.